1. Field of the Invention
The present invention relates to a noise control apparatus for use on top of a straight upright sound barrier provided to reduce noises emanating from road, railway, factory, etc.
2. Description of Related Art
FIG. 1 shows a typical conventional noise control apparatus for use on top of a straight upright sound barrier or screen, existing or newly erected, which will be referred to as "main sound barrier" for the convenience of the explanation hereinunder). As seen, the noise control apparatus comprises a main sound barrier 100, a first additional screen 101 installed on top of the main sound barrier and tilted towards a sound source, and a second additional screen 102 installed atop the main sound barrier and tilted away from the sound source (namely, towards protected area). As will be understood from FIG. 1, the main sound barrier 100, first and second additional screens 101 and 102 form together a structure having a Y-shaped cross section. This Y-shaped structure reduces noise rather more effectively than a straight upright sound barrier or screen having a same height.
The conventional Y-structure of sound barrier has been required for an improved capability of sound attenuation and further compact and lightweight design. Especially, since there is a regulation in Japan that the upper portion of the sound barrier of this type for use along the roadway should not overhang more than 0.25 m over the road surface, the sound barrier structure is required to be more compact while maintaining the improved capability of sound attenuation.